1. Field of the Invention
The present invention relates generally to current generation circuitry and, in particular, to a current generating circuit which does not rely upon resistors to control the current magnitude and which has a temperature coefficient which is advantageous in many applications.
2. Background Art
Referring to the drawings, FIG. 1 is a schematic diagram of a conventional current generator circuit which utilizes both MOS and bipolar circuit components. Circuit 6 includes a pair of PMOS transistors 10A and 10B connected as a current mirror. Since transistors 10A and 10B are the same size, the drain-source currents for the two transistors are the same. A second pair of cascode-connected NMOS transistors 12A and 12B are connected in series with transistors 10A and 10B, respectively. Transistors 12A and 12B operate to maintain their respective source voltages, VA and VB, at the same level.
PNP transistors 14A and 14B are connected in series with transistors 12A and 12B and thus conduct equal currents. Typically, transistors 14A and 14B are parasitic substrate transistors that are present in many circuits fabricated using conventional MOS processes. Transistor 14B has an emitter area A2 which is larger than the emitter area A1 of transistor so that the base-emitter voltage 14B is smaller than the base-emitter voltage of 14A. The difference in base-emitter voltages xcex94VBE is given by the following equation:
xcex94VBE=(kT/q) ln (A2/A1)xe2x80x83xe2x80x83(1)
where k is Boltzmann""s constant, q is electronic charge and T is temperature in Kelvin.
Since xcex94VBE is the voltage drop across resistor R, the current flow I through resistor R is as follows:
I=[(kT/q) ln (A2/A1)]/Rxe2x80x83xe2x80x83(2)
The output Iout of current generator 6 is provided by a third PMOS transistor 10C connected to have the same gate-source voltage as transistors 10A and 10B. Iout can be made to differ from I by adjusting the channel width of transistor 10C relative to the channel width of transistors 10A and 10B.
One shortcoming of the FIG. 1 biasing circuit is due to the fact that the value of resistor R, which determines the output current Iout, is not well controlled. In a typical CMOS process, resistor R is made of diffusion or poly silicon. Neither of these materials provides a tight control on the resistor value, which could vary xc2x130% from the nominal value.
If a circuit being biased by the FIG. 1 current generator circuit requires a certain amount of minimum current, Inom, the current generator must be capable of providing 1.3 (Inom) to ensure that current Inom will be provided where the resistance is 30% larger than the nominal value. At the same time, if the resistance turns out to be 30% less than the nominal value, then the current generator will provide (1.3)(1.3) Inom or 1.7 Inom. This is 70% more current than the current generator was nominally required to provide.
The present invention addresses the above-noted shortcomings of the prior art by providing a current generator circuit with an output current which is more precisely controlled. Thus, unnecessary power consumption is substantially reduced. In addition, as will be explained, the current generator circuit disclosed herein is capable of enhancing the settling time of amplifier circuits which are biased by the circuit. These and other advantages of the present invention will become apparent to those skilled in the art upon a reading of the following Detailed Description of the Invention together with the drawings.
A current generator circuit which provides an output current having a stable absolute value and a temperature coefficient which, when used to bias an amplifier, provides reduced settling time and optimum power consumption. A first MOS transistor conducts a current related to the output current and is biased to operate in the linear region. A second MOS transistor, having gate and source electrodes which are connected to the gate and source electrodes of the first MOS transistor, is biased for saturation region operation. The second MOS transistor also conducts a current related to the output current and is typically equal to the current of the first MOS transistor.
The current generator circuit preferably further includes a pair of bipolar transistors operating at different current densities to as provide different base-emitter voltages. The bipolar transistors are connected relative to the first MOS transistor so that the drain-source voltage of the first MOS transistor is equal to the difference between the base-emitter voltages. The output current is more stable than that provided by prior art current generator circuits and provides a current which is approximately proportional to TD0.5, with T being temperature in degrees Kelvin.